Study Finds Baker’s Law Helps Invasive Plants Spread

Imagine one seed falling onto an exposed roadside verge, hundreds of kilometres from the place where its parent plant once grew. Not another plant of the same kind can be seen nearby.

For the majority of flowering plants, that would be the end of the line. Flowers generally require pollen from a second plant of the same species to produce seeds, leaving a lone newcomer unable to reproduce.

Some plants, however, bypass that limitation. By producing seeds using their own pollen, a single surviving plant can establish an entire population.

New research from Bengaluru provides compelling evidence that this capacity is among the characteristics distinguishing an innocuous introduced species from one that becomes dominant.

One plant can establish a new population

Most flowering plants depend on cooperation, with pollen carried between separate individuals.

There is a clear evolutionary benefit to this arrangement. Exchanging pollen among plants maintains genetic diversity, which generally leads to healthier offspring.

A smaller group of species can reproduce independently. They can form seeds using their own pollen, or in some cases without any pollen, enabling just one plant to found a colony far from its original range.

The principle known as Baker’s law

This is not a newly proposed theory. In the middle of the twentieth century, biologist Herbert Baker suggested that plants able to reproduce without a mate would be more successful invaders.

The logic is persuasive: when a single dispersed seed can create a population, self-compatible species have a considerable advantage on reaching unfamiliar territory.

For many years, the proposition remained largely theoretical, as demonstrating it under natural conditions was challenging. Establishing how dozens of plant species reproduce requires painstaking research.

Testing Baker’s law in daisies

To examine Baker’s proposal, two researchers from the Indian Institute of Science (IISc) studied the daisy family, which botanists call Asteraceae.

Saskya van Nouhuys, an associate professor at the Centre for Ecological Sciences (CES), worked with Narashiman Nagendra Rao, a former master’s student in her laboratory.

The daisy family offered a suitable group to investigate. It is among the world’s largest plant families and includes some of the most infamous weeds on the planet.

The researchers compared 28 species altogether, including 11 recognised invasive species.

A further eight were non-native arrivals that had not become invasive, while the remaining nine had long been native to the area.

Monitoring hundreds of plants

The field research continued for an entire year. Across Karnataka and Tamil Nadu, the team collected plants from untidy, disturbed locations, including roadsides and empty plots.

They grew seeds and cuttings from at least five plants of each species on campus.

Several native species would not grow successfully in these conditions, so the researchers examined them in the wild instead.

By the end of the study, the team had assessed approximately 900 plants.

What microscopy revealed

The crucial test followed. Scientists compared flowers allowed to self-pollinate with flowers pollinated in the conventional manner, before determining which seeds were viable enough to germinate.

Fluorescence microscopy allowed the team to investigate in even greater detail.

It enabled the researchers to establish whether each seed resulted from self-fertilisation or apomixis, a form of seed production that takes place without fertilisation.

This difference is more significant than it may first appear. A seed formed without pollen is a precise genetic copy of its sole parent, allowing one plant to clone a population with almost no limit.

The team could therefore classify every species according to its reproductive strategy.

Every invasive plant reproduced independently

The result was striking: all 11 invasive species were capable of reproducing alone, without needing another plant.

Most native species and non-invasive foreign arrivals could not do so. They remained self-incompatible and required pollen from a different plant in order to produce seeds.

“Uniparental reproduction is a conceptually simple trait. It has been exciting to see such clear evidence of its advantage for invasive species,” said Professor van Nouhuys.

Two invasive plants altered their breeding strategy

Two species extended the finding even further. Ageratum conyzoides and Bidens pilosa are among the most aggressive weeds in the entire sample.

In their native Mexico, both species are largely self-incompatible and depend on pollen from other plants. In India, where they are introduced species, they have moved towards fully independent reproduction.

That comparison makes the evidence particularly persuasive: the same species display contrasting reproductive behaviour on two continents, depending on whether they are in their native range or spreading elsewhere.

A change while invading new territory

This change was what particularly surprised the researchers. It indicates that, as the plants expanded across India, the individuals capable of self-fertilisation were those most likely to succeed.

“Before our experiments, the idea of reproductive strategies shifting during invasion seemed like a very far-fetched idea to me and I thought that previous evidences of such shifts were very rare occurrences,” said Narashiman.

“The results from our experiments and that of our collaborators absolutely baffled me.”

Improved ways to identify invasive plants

The study also has a practical implication. Schemes that assess incoming plants for their invasion risk generally consider features such as growth speed and climate suitability.

The researchers argue that reproductive strategy should also be included in these assessments. Establishing whether a plant can reproduce alone could help identify the next major invader before it becomes established.

The issue becomes more pressing each year. Trade and travel continue to transport plants worldwide, extending the list of species that could potentially become invasive.

Invasive plants reshape ecosystems

The consequences extend well beyond a single weed beside a road. Once an invasive species becomes established, plants already present in that environment commonly begin to decline.

The effects then move through the wider ecosystem. Insects that rely on those plants lose a food source, and birds higher up the food chain are affected in turn.

“Invasive species are called invasive for a reason,” said Professor van Nouhuys.

“They establish and then flourish in a new location. When this happens, existing species decline or disappear entirely, which changes the whole landscape.”

One seed landing at the side of a road, it seems, can mark the first stage of a far broader transformation.